The present invention relates to the production of sulfur trioxide, sulfuric acid and oleum, and especially to such processes wherein the conversion of sulfur dioxide to sulfur trioxide takes place at superatmospheric pressure and at least a portion of the effluent from the converter is cooled to directly condense sulfur trioxide.
The well-known process for the production of sulfuric acid involve three basic steps: burning sulfur to form sulfur dioxide, converting the sulfur dioxide catalytically to sulfur trioxide by reaction with air and absorbing the sulfur trioxide from the effluent from the converter in an absorption tower. In such processes, the unreacted sulfur dioxide content of the converter effluent has caused problems in that the remaining sulfur dioxide represents an unacceptable emission to the atmosphere. Accordingly, double effect systems have been used wherein the unabsorbed vapor from the absorption tower is mixed with air and again catalytically converted, with the effluent from the second convertor passed through a second absorption tower. Such processes involve high capital costs and generally product only sulfuric acid as product.
High pressure processes have also been proposed, but because of capital requirements have not been commercially practiced widely, wherein either the feed to the burner or the feed from the burner to the converter are pressurized so as to improve the yield of sulfur trioxide and thereby reduce the unreacted sulfur dioxide content of the converter effluent.
Various sources of sulfur dioxide are now becoming available from desulfurization of flue gases, and some such sources produce sulfur dioxide of greater than 12% concentration. Thus far, such concentrated sulfur dioxide is sometimes reduced to sulfur, which in turn may be fed to the burner of a sulfuric acid plant. The sulfur dioxide can also be diluted and fed to a conventional converter, but such processes have all the disadvanatages of a conventional sulfuric acid process.
Oleum, that is sulfuric acid containing sulfur trioxide in excess of the stoichometric amount to react with water to form sulfuric acid, is conventionally produced by absorbing sulfur trioxide in sulfuric acid. Such direct absorption has the disadvantage that the greatest concentration practically achievable is about 25%. Higher concentrations are achieved by desorbing sulfur trioxide from 20 or 25% oleum, condensing the sulfur trioxide and then mixing condensed sulfur trioxide with 20% oleum. Liquid sulfur trioxide is also conventionally produced by stripping 20% oleum and then condensing the stripped gases. All such processes have high capital and energy requirements, and are generally limited to the production of one or a limited number of the various products desired.
U.S. Pat. No. 4,213,958 to Cameron et al. (July 22, 1980) discloses a process for the production of sulfuric acid as the main product and, optionally, liquid sulfur trioxide as a secondary product. In this process inlet air is pressurized to about 20 atmospheres after drying and the conversion step is operated at such pressure. As a result, the converter effluent, after a 99 or 99.7% conversion, can be cooled to directly condense a portion of the sulfur trioxide, along with a portion of the unreacted sulfur dioxide. The sulfur trioxide remaining in the vapor phase is passed through a conventional absorber, operating however at 20 atmospheres pressure, and the sulfuric acid formed is depressurized by passage through a turbine before passage through an air drying tower where it passes in mass exchange with incoming reaction gases. As a result, the sulfur dioxide content of the sulfuric acid passing through the turbine is released to the reaction gases, while the water vapor in the reaction gases is absorbed in the sulfuric acid. Sulfuric acid from the drying tower is stored, with a portion taken as product and a remaining portion pumped back to 20 atmospheres pressure by a pump driven at least in part by the turbine and returned to the high pressure absorption tower. While such a process has additional flexibility in being able to produce sulfur trioxide directly and to produce sulfuric acid, it has the disadvantage of requiring extremely high pressures and resulting high work requirements to drive both the air compressors and the acid pump to the extent of inefficiencies in the turbine and the pump. Furthermore, at the high pressures indicated, the sulfur dioxide content of the liquified sulfur trioxide renders it a less desirable product than sulfur trioxide formed by stripping oleum and recondensing the vapor.